ASCA operation

Dear ASCA Team members and ASCA users:
We again experienced malfunction of ASCA attitude control system
on November 18 and found that the malfunction happened at the
time of maneuver to NGC3516 at 23:40 (UT) on Nov. 17 when the
ASCA star tracker did not catch the target stars in the tracking
areas at the end of maneuver and the optical axis was about 3
degrees offset. During the contact passes on Nov. 18 we examined
and solved the problem. The phenomena was similar with the
trouble on September 7 when ASCA transferred into the Safe Hold
Mode.
At preset, we have been operating ASCA by using X- Y- and S1-
gyroscopes (Z-gyro being stand-by for emergent transfer to a safe
hold mode). HK data for urrent monitor indicated that the motor
current of one of the three gyros (S1) had become unstable since a
few days before the time of malfunction. It seems that the rotation
speed of the S1- gyro became unstable from the time,which caused
the star tracker system failed to catch up the target stars in the
tracking field at the end of the maneuver from 3C390.3 to
NGC3516. This is suspected to be due to the small change of pulse
weight from the setted parameter (during pointing or small
maneuver, the small error in pulse weight do not affect to the
control but the error in pulse-weight is accumulated during the
long maneuver).
When we turned off and restarted the motor of S1 gyro, the ACS
system have started to function properly. We started regular GO
observations after the contact passes on Nov. 20 from PSR00437-
4715 along the weekly schedule. We are now investigating
methods to improve the ACS system for operating ASCA in more
stable. We are now planning to take a few days for restructuring
the ACS system in early December. After then we will tune the
system along with the GO observations.
Because of this events, we were not able to observe NGC3516,
A1722, A1704 and half of PSR00437-4715. The new weekly
schedules will be announced seperately including the recovery of
those lost observations and the ASCA maintenance. The ASCA
long-term timeline will be also updated.
Regards,
Fumiaki Nagase